1. Field of the Invention
This invention relates to photopolymerizable compositions. More particularly, this invention relates to compositions which comprise an organic material having epoxide functionality and another organic material having hydroxyl functionality. The compositions further contain an aromatic sulfonium or iodonium complex salt as photoinitiator and can be cured by exposure to actinic radiation or electron beam irradiation.
2. Description of the Prior Art
Although photopolymerizable epoxy compositions comprising epoxy resin materials and photosensitizers have been desired for some time, those which have previously been proposed suffer from one or more drawbacks. Thus, in U.S. Pat. No. 3,074,869 there are disclosed photosensitive epoxy compositions containing a nitrosoamine as photosensitizer. Compositions of this type require relatively long exposure to a high intensity light source to produce complete polymerization.
In U.S. Pat. Nos. 3,205,157 and 3,708,296 there are disclosed photosensitive epoxy compositions containing respectively aryldiazonium salts and aryldiazonium salts of halogen-containing complex anions. Such compositions have limited usefulness because they have poor thermal stability, and because nitrogen is evolved during photopolymerization causing pinholes and bubbles in coatings of the composition.
When these known aryldiazonium salts are used to induce polymerization of oxetanes, or mixtures of oxetanes with epoxy resins, e.g., as described in U.S. Pat. No. 3,835,003, the same types of problems are encountered. Although several patents describe various techniques for stabilizing mixtures of diazonium salts and epoxides, such techniques are not satisfactory for several reasons. For example, the increase in stability which is obtained is measured only in days. Also, the addition of stabilizers contaminates the compositions with non-reactive material which softens the resulting products and also reduces the rate of photocure. See, e.g., U.S. Pat. Nos. 3,711,390; 3,711,931; 3,816,278; 3,816,280; 3,816,281; 3,817,850 and 3,817,845.
In U.S. Pat. No. 3,450,613 there is described another photopolymerizable epoxy composition comprising the reaction product of an epoxy resin prepolymer and an ethylenically unsaturated organic acid, a photosensitizer, and optionally polyfunctional acids or bases. This composition on exposure to ultraviolet light gels by reason of the photoinduced polymerization of the ethylenically unsaturated portion of the reaction product. Completion of the cure of the composition is effected by heating to bring about reaction of the epoxy resin portion of the composition. Such compositions have limited usefulness because of the requirement of both light and heat to effect complete polymerization of the composition. The composition furthermore is oxygen sensitive and has poor thermal stability.
There is also a need for a thermally stable, one-part, photocurable epoxy composition which provides a flexible coating. Such has not heretofore been provided.
The present invention provides novel photopolymerizable compositions comprising organic material having epoxide functionality, another organic material having hydroxyl functionality, and a complex salt photoinitiator. The compositions may be photocured by exposure to light or by exposure to electron beam irradiation.
Although reaction between epoxides and aliphatic hydroxyl groups has been known, such reaction has been carried out at elevated temperatures by heating the reactants in the presence of basic or acid catalysts. For example, U.S. Pat. Nos. 2,731,444; 2,830,031; 2,914,490; and 3,826,669 describe the thermal reactions of a variety of epoxides with different types of polyols including copolymers containing reactive hydroxyl groups. Although such reactions proceed very slowly in the presence of a catalyst at room temperature, elevated temperatures (e.g., 125-250.degree. C.) are required in order to get reasonable reaction rates and useful properties. Further, those mixtures described, in the patents, which contained acid catalysts were not stable at room temperature.